This master thesis presents an experimental study of the metallurgical and mechanical compatibility of a heterogeneous cladding in order to develop new methods to repair hydraulic turbine runners damaged by fatigue. The heterogeneous cladding studied consists of duplex stainless steel E2209 deposited on S41500 low carbon martensitic stainless steel. The second objective of this experimental study focuses on the effect of the hammer peening process on the tensile residual stresses generated during the deposition of the filler metal.
This study is motivated by the problem of repairing the damage caused by fatigue without resorting to post weld heat treatment. In fact, the S41500 low carbon martensitic stainless steel and its cast version CA6NM, used for the manufacture of several hydraulic turbine runners, requires a post weld heat treatment to soften the freshly formed martensite. The repairs to be made in the turbine pit to limit the downtime of electricity production, make it difficult to achieve a post weld heat treatment, so the use of a homogeneous filler metal for repairs is proscribed.
Various plates simulating the repair of a crack have been cladded by varying the heat input or the chemical composition of the shielding gas in order to study the effect of these two variables on the microstructure and the mechanical properties of the cladding. From these plates, tensile tests, toughness tests and microhardness profiles were carried out in order to characterize their mechanical behavior. The contour method was used to evaluate the level of residual stresses in the cladded region. This technique also allowed evaluating the effect of hammer peening on the residual stresses level.
Metallographic samples were taken from the different combinations of parameters to characterize the fusion zone microstructure, but also the fusion line and the heat affected zone of the base metal. The proportion of austenite and ferrite constituting the duplex alloy was evaluated by image analysis and backscattered electron diffraction analysis (EBSD). Images obtained by optical microscopy confirmed the ferritic solidification of the duplex alloy and the solid-state transformation of ferrite into austenite under different morphologies during cooling. The presence of nitrides within the ferritic matrix has also been observed. The scanning electron microscope (SEM) was used to investigate the presence of intermetallic compounds in the fusion zone. Precipitates were indeed observed at the austenite-ferrite interfaces, but the analysis of their chemical composition using energy-dispersive X-ray spectroscopy (EDS) could not confirm that it was σ-phase.
| Date | 9 Jan 2020 |
|---|
| Original language | French |
|---|
| Awarding Institution | - École de technologie supérieure
|
|---|
| Supervisor | Tan Pham (Supervisor) & Stéphane Godin (Co-supervisor) |
|---|
Hudon, J.-B. (Author),
Pham (Supervisor) & Godin (Co-supervisor),
9 Jan 2020Student thesis: Master's thesis › Master in Engineering: Mechanical Engineering